1. Field of the Invention
The present invention generally relates to an ATM (asynchronous transfer mode) switching system. More specifically, the present invention is directed to such an ATM switching system for managing control information on an ATM switch for switching cells with employment of a virtual path identifier (VPI) and a virtual channel identifier (VCI).
2. Description of the Related Art
The aforementioned conventional ATM switching system is mainly arranged by, as represented in FIG. 1, the ATM switch 20 for switching the cells and the CPU (central processing unit) 30 for controlling this ATM switch 20 under software control. In this ATM switching system, the call control unit 31 of the CPU 30 analyzes the signaling information such as call setting and call releasing issued from the subscribers (terminals) 41 and 42, or another switching system (not shown), and then the resource managing unit 32 allocates and releases the resources for the call, e.g., the virtual paths (VP), the virtual channels (VC) and bands provided on the input side and the output side based on the analytical results obtained from the call control unit 31. Under control of the call control unit 31, the switch control unit 33 controls the ATM switch 20 based on the resource information to set, change and release the connections. At this time, the switch control unit 33 registers various sorts of control information used to control the ATM switch 20 into the ATM switch control information management tables (will be abbreviated as "management tables" hereinafter) 34 and 35 for management purposes. The management tables 34 and 35 own such hierarchial structures that are retrievable based upon the numbers of the designated virtual path and virtual channel, namely the values of these VPI and VCI (will also be called as "routing bits" altogether).
The respective cells for transporting the transfer information (i.e., user information) between the subscribers 41 and 42 are entered into the switching system by setting the VPI and VPC to the header, which have been designated when the connection is established. In this switching system, the management tables 34 and 35 are retrieved based on these VPI and VCI so as to acquire the route number of the ATM switch 20 on the output side, and also the necessary control information of the VPI and VCI, the header of the cell is rewritten based on those control information for the cell switching.
Since the management tables used to store the control information about the ATM switch may be directly retrieved based on the values (routing bit values) of the VPI and VCI in the switch control information managing method of the above-described conventional ATM switching system, the management tables with a large memory capacity capable of storing therein the maximum routing bits of these VPI and VCI should be prepared. For instance, in case of the Broadband ISDN (integrated services digital network), since 12 (otherwise 8) bits (routing bit) are allocated to the VPI and 16 bits are allocated to the VCI, the resultant entry size of the management table requires 2.sup.28. This may cause an economical problem. To avoid such a problem, it is conceivable to establish a limitation in the actually used numbers of VPI and VCI, namely the actually used bit numbers, as a result of negotiations with the user. For example, when four sets of VPIs and 1000 sets of VCIs are employed, the routing bits of 2 and 10 may be allocated thereto. However, because the use bit positions at this time are allocated from the least significant bits (LSB) with respect to both of these VPI and VCI, taking account of continuities of the values used to retrieve the management table, arbitrary values within the bit numbers (12, 16) in the respective specifications cannot be set (see Japanese Publication "INTRODUCTION TO B-ISDN" published by the Japan ITU Foundation, Volume 1, 1992, pages 130 to 131 , Section 5.1 [2] (a) Routing Bits (VPI, VCI), and pages 140 to 141, Section 5.3 [3] (c) Routing Field (VPI, VCI).
Accordingly, an object of the present invention is to provide an ATM switching system capable of reducing a storage capacity of a management table for storing control information on an ATM switch without restricting bit values of VPI and VCI, even when there is a limitation in quantities of these VPI and VCI used to designate a cell transfer route.